Simple control of industrial AC motors requires only the connection and disconnection of the motor windings from a source of AC power. This may be performed by an electromagnetic relay (contactor) which switches the large motor currents in response to a smaller current energizing its electromagnetic actuator. An `ON` pushbutton switch, when pressed, allows current flow which closes the contactor. An auxiliary set of latching contacts in the contactor hold the contactor in the `ON` state even when the `ON` pushbutton switch is no longer closed. A STOP pushbutton switch interrupts the current flowing through the electromagnetic actuator to allow the contactor to open turning the motor off. Often the contactor is associated with an overload relay, the latter which opens the contactor if motor currents exceed a predetermined limit, indicating an overload.
With advances in solid state electronic devices, motor controllers have been developed which provide additional and more sophisticated control of the motor. In such motor controllers, the power to the motor passes through a set of solid state switches, such as thyristors, which may modulate the AC power to adjust the motor voltage and current. Such modulation, for example, can be used to "soft start" the motor by slowly increasing the voltage to the motor, as the motor is started. A soft start reduces current consumption and torque during start-up and thus avoids excessive current demands on the electrical power distribution system.
Such motor controllers can also provide a dynamic braking action by applying DC or alternating current to the motor so as to slow down the motor at a faster rate than would occur simply by disconnecting power.
With the availability of microprocessors, additional motor functions may be implemented. Thus, even for smaller motors, it is typical to find the simple start and stop pushbutton augmented by additional controls providing commands such as jog, change motor direction and change motor velocity.
Frequently, it is desirable that a motor be controllable from a number of different places for the convenience of the machine operators. In automated processes, it is also desirable that the motor controller also receive commands from a programmable logic controller (PLC) or other industrial computer.
Previously, multiple control sources, either pushbuttons or a combination of pushbuttons and other controllers, have been connected together to provide commands to a single motor controller. Conflicts among these multiple control sources are prevented by permitting the motor controller to accept certain command types only from a single predetermined command source. For example, a `JOG` command might be accepted only from a single pushbutton panel. Changing the command source for a given command function requires reprogramming of the motor controller by the user.
Although this approach significantly limits flexibility inherent in multiple command sources and increases the complexity of configuring the controller, it has the important advantage of ensuring a clear control hierarchy and eliminating the possibility of erroneous interpretation of competing conflicting commands.